In order to convert an input voltage with a frequency f1 into an output voltage with a frequency f2 multilevel converter can be used.
With reference to FIG. 1A, a multilevel converter 1 has inputs 2, outputs 3 and capacitors 4 between the inputs 2; all the capacitors 4 have the same capacitance. The capacitors 4 are used to divide the input voltage in steps that are then combined to create a desired sinusoidal step modulated output voltage; in FIG. 1B reference 5 indicates the grid voltage, reference 6 indicates the desired voltage and reference 7 indicates the output voltage considering a connection to the grid via an impedance (reactor or transformer).
When the capacitors 4 have different capacitance (also small differences can be relevant and could for example be due to defecting manufacturing or connections) the voltage steps are different from one another and the control of the output voltage can become problematic, because it would need a sophisticated control of the voltage distribution between the capacitors.